This invention is directed to a lightweight tire support. This invention is particularly directed to a method and composition for preparing a lightweight tire support that is temperature stable and has load-bearing capability. More particularly, the invention is directed to a polyurethane-polymer composition that can be used to prepare a tire support and to a method of preparing a tire support from a polyurethane-polymer composition of the invention.
A tire support for a vehicle tire is a support suitable for being mounted on a wheel rim inside of a tire. A tire support is a precautionary device against a tire losing air pressure, and it is intended to bear the weight of a vehicle if a tire loses partial or total air pressure to the extent that loss of vehicle control or irreparable damage to the tire might occur by continued use of the vehicle without a tire support.
A tire support is desirable because it can replace a spare tire to enable a vehicle to continue traveling to a service facility where inflation, repair, or replacement of an at least partially-deflated tire can be accomplished. This is advantageous for manufacturing small vehicles that lack the capacity for a spare tire and a jack, for reducing the overall weight of a vehicle to improve fuel economy, and for reducing the likelihood of additional vehicle damage when a vehicle continues after a tire loses air pressure.
Several devices intended for use as a tire support are known. Examples of tire supports are described in U.S. Pat. Nos. 4,248,286; 4,318,435; 4,418,734; 4,461,333; 4,592,403; 5,363,894; and 5,891,279.
A variety of materials, such as metals, rubbers, and plastic elastomers, have been used to prepare known tire supports. Tire supports made of metal are less than desirable because these types of devices do not withstand impacts well and can be fragile. Moreover, any failure in such a device can lead to a rapid and complete destruction of the tire within which the support is included.
Tire supports prepared from rubber are also less than desirable. Tire supports made from rubber generally weigh at least 7 kg and can weigh as much as 25 kg, which can adversely affect vehicle fuel economy. One type of tire support prepared from rubber includes a vulcanized mix of natural or synthetic rubber. This type of tire support can be incompatible with a tire and can prematurely degrade. For example, travel on a tire containing such a support can produce rapid and premature heating of the tire and of the support.
A variety of plastic elastomers have also been disclosed as useful for preparing a tire support. For example, elastomer compositions, such as a polyurethane, a latex, and a synthetic plastic, have been cited as useful for preparing a tire support. This type of support can have problems with, for example, temperature instability, with cracking, and with processing. Thus, the known plastic supports are less than desirable.
None of the known materials can provide a tire support with good load-bearing performance, temperature stability, and low-mass structural design. Also, methods known to be useful for preparing tire supports from known materials can be prohibitively expensive for commercial application and can require less than desirable processing times for making each support.
It would be desirable to prepare a support from a plastic elastomer, such as a polyurethane polymer, because materials needed to prepare a plastic elastomer are generally relatively inexpensive compared to other materials, such as rubber, and because a plastic elastomer can be lighter (that is, have less weight) than other materials such as rubber, which can be useful for improving fuel economy of a vehicle compared to the fuel economy of vehicles using known supports.
One challenge in making a tire support from a plastic elastomer is that an elastomer""s properties are temperature dependent. That is, elastomers are generally ductile at room temperature (about 22xc2x0 C.) but may not demonstrate similar characteristics as they are exposed to a range of temperatures, especially at high temperatures of greater than about 100xc2x0 C. Because a tire support will be placed on vehicles that will be exposed to a variety of temperatures such as winter temperatures below 0xc2x0 C. and summer temperatures of as high as 50xc2x0 C. depending on geographic location and because a tire support can be exposed to an operating temperature of, for example, 120xc2x0 C., a plastic elastomer should be relatively stable over a range of temperatures to provide desirable performance.
Another challenge in making a tire support from a plastic elastomer is that the elastomer""s properties should be effective to provide a tire support with sufficient durability so that the tire support can provide desirable performance and be used for its intended purpose. That is, the support should be durable enough to be useful for supporting a vehicle""s weight during run-flat conditions, which is known as being load bearing. This challenge becomes greater as the weight of a tire support is reduced to produce a lightweight tire support, for example, a tire support weighing no more than about 8 kg, because a polymer generally loses tensile modulus, which can lead to polymer deformations such as high frequency flexing, bending, and buckling of the polymer. These deformations can adversely affect durability. It is well known that a lightweight tire support is generally more prone to structural failure because a polymer becomes more prone to cracking and structural degradation when its mass is reduced.
It would be desirable to prepare a support with good load-bearing performance, good temperature stability and low-mass structural design. Specifically, there is a need to provide a tire support of a suitable polymeric composition that achieves these performance criteria. Additionally, there is a need for an efficient process for preparing a support from that composition. More specifically, there are needs for a polyurethane-polymer composition that can form a suitable tire support and for a process that can improve at least one of production time and manufacturing economics of a tire support.
According to the invention, a polyurethane-polymer composition suitable for preparing a lightweight tire support includes at least one isocyanate, at least one polyol, and at least one chain extender. At least one isocyanate is present in an amount effective to react with at least one chain extender to provide a hard-segment polymer and in an amount effective to react with at least one polyol to provide a soft-segment polymer. The hard-segment polymer and the soft-segment polymer are present in an amount effective for providing a tire support prepared from the composition with temperature stability and load-bearing capability.
Load-bearing capability can be determined by evaluating a tensile modulus for a test sample prepared from a polyurethane-polymer composition of the invention. In one embodiment, a polyurethane polymer""s tensile modulus is at least 25 mPa at 80xc2x0 C. but no greater than 60 mPa at 80xc2x0 C. This measurement can be made according to ASTM D638.
Temperature stability can be determined by evaluating a change in tensile modulus over a temperature range for a test sample prepared from a polyurethane-polymer composition of the invention. In one embodiment, a polyurethane polymer""s tensile modulus remains substantially unchanged over a temperature range of xe2x88x9250xc2x0 C. to 150xc2x0 C. This measurement can be made according to dynamic mechanical spectroscopy.
In one embodiment, a polyurethane-polymer composition of the invention includes at least two polyols including a polypropylene oxide and an ethylene oxide-capped polyol; at least one of an aliphatic amine chain extender, an aromatic amine chain extender, and a modified amine chain extender; and at least one isomer of diphenylmethanediisocyanate. The composition includes isocyanate in an amount effective to react with the chain extender to provide a hard-segment polymer in an amount of at least 25 weight percent and no greater than 45 weight percent based on the total weight of the composition. A tire support prepared from this composition has a weight of no greater than 8 kg, a tensile modulus of at least 30 mPa at 80xc2x0 C. and no greater than 50 mPa at 80xc2x0 C., and a change in tensile modulus of no more than 15 percent over a temperature range of xe2x88x9250xc2x0 C. to 150xc2x0 C.
Also according to the invention, a lightweight tire support can be prepared from a composition of the invention. A lightweight tire support generally includes a hard-segment polymer including a reaction product of an at least one first isocyanate and at least one chain extender and a soft-segment polymer including a reaction product of at least one second isocyanate and at least one polyol. The at least one first isocyanate and the at least one second isocyanate can be the same or different. The tire support includes the hard-segment polymer and the soft-segment polymer in an amount effective for providing the tire support with load-bearing capability and temperature stability.
A lightweight tire support of the invention can be prepared by injecting an isocyanate-side stream into a processing mold designed to mold a tire support; injecting a polyol-side stream into the processing mold; and reacting the isocyanate-side stream and the polyol-side stream in the processing mold. The isocyanate-side stream includes at least one isocyanate, and the polyol-side stream includes at least one polyol and at least one chain extender. A tire support prepared by this method includes a hard-segment polymer and a soft-segment polymer in an amount effective for providing the tire support with temperature stability and load-bearing capability.